1. Field of the Invention
The present invention relates to a semiconductor laser device used in an optical disc system, optical communications, etc., and particularly to a semiconductor laser device wherein a refractive index distribution is asymmetrical with respect to an active region.
2. Description of the Related Art
In recent years, a large-capacity and portable writeable optical disc system has become exponentially common as an external memory device or the like for a personal computer. The development of a semiconductor laser device high in optical output efficiency and good in optical characteristics and temperature characteristics is essential to the size reduction and portability of the optical disc system.
One method of increasing slope efficiency of a semiconductor laser and enhancing kink level and temperature characteristics or the like thereof has been proposed. In an n-type (AlxGa1-x)yIn(1-y)P cladding layer (“n type” and “p type” conductivity type are hereinafter denoted as “n-” and “p-” respectively) and a p-(AlxGa1-x)yIn(1-y)P cladding layer, both employed in a semiconductor laser producing red light using, for example, AlGaInP, composition ratio x of the n-cladding layer and composition ratio x of the p-cladding layer are set to different values. The refractive index of the n-cladding layer is set higher than that of the p-cladding layer, whereby light distribution is biased toward the n-cladding layer side and has reduced light absorption (refer to, K. Shigihara et Al., “High-power 980-nm Ridge Waveguide Laser Diodes Including an Asymmetrically Expanded Optical Field Normal to the Active Layer”, IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 38, No. 8, AUGUST 2202, pp.1081-1088 (FIG. 6), for example).
In a GaN-based laser producing blue-violet light, it has been disclosed that refractive indices of optical guide layers sandwiching an active layer having a multi-quantum well structure asymmetrical to shift peak position of light intensity distribution to the P side relative to the position of an active layer. The active layer is prevented from deteriorating and the reliability at high optical output is enhanced (refer to Japanese Patent Laid-open No. 2002-299768 (eighth column, FIG. 9), for example).
There has also been disclosed an example in which the thickness and a refractive index distribution of an n-cladding layer on the n-GaAs substrate, and the thickness and a refractive index distribution of a p-cladding layer having a ridge form at part thereof, provided on the opposite side of the n-GaAs substrate with respect to an undoped InGaAs active layer are respectively asymmetrical. According to this configuration, the influence of a refractive index distribution in a width direction of a ridge portion, which is produced due to the difference in refractive index between the ridge portion and an outer side of the ridge portion, upon the light in the waveguide is reduced. The occurrence of high-order modes is prevented, thereby making it possible to prevent the occurrence of a kink at low output so an increase in output can be achieved (refer to Japanese Patent Laid-open No. 11-233883 (FIGS. 1, 2, 4 to 6 and 8), for example corresponding to U.S. Pat. No. 6,285,694).
A example has been disclosed wherein in an AlGaInP semiconductor laser element, an n-Al0.66In0.34P cladding layer 3, an undoped (Al0.15Ga0.85)0.66In0.34P optical guide layer 4, an undoped GaInAsP active layer 5, an undoped (Al0.15Ga0.85)0.66In0.34P optical guide layer 4, an MQW structure 6 (p-Al0.66In0.34PMQW barrier layer, p-(Al0.15Ga0.85)0.66In0.34PMWQ well layer), and a p-Al0.66In0.34P cladding layer 7 are provided, thereby improving device characteristics, such as threshold current and characteristic temperature (refer to Japanese Patent Laid-open No. 2001-24285 (FIG. 4), for example corresponding to U.S. Pat. No. 6,542,528).
An example has been disclosed wherein a semiconductor laser epitaxial crystal laminated body having upper/lower cladding layers sandwich an active layer having a quantum well structure, and AlGaAs low refractive index layers are respectively provided between the upper/lower cladding layers and upper/lower guide layers provided between the cladding layers and the active layer, and which have a refractive index lower than each of the upper/lower cladding layers, are made asymmetrical in Al composition and thickness (refer to Japanese Patent Laid-open No. 8-195529 (FIGS. 9 and 10), for example).
Further, an example has been disclosed wherein a semiconductor laser element having a configuration in which an n-(Al0.7Ga0.3)0.5In0.5P lower cladding layer and a p-(Al0.7Ga0.3)0.5In0.5P first upper cladding layer are disposed up and down with a multi-quantum well active layer interposed in between, and a p-(Al0.7Ga0.3)0.5In0.5P second upper cladding layer is disposed on the first upper cladding layer interposing a protective layer of p-Ga0.5In0.5P in between, has an active layer disordered region in which a quantum well active layer is disordered, at an end face of a resonator and in the neighborhood thereof (refer to Japanese Patent Laid-open No. 2001-15864 (twelfth column and FIG. 4), for example).
A conventional semiconductor laser device is a semiconductor laser wherein n- and p-cladding layers sandwich an active layer having a quantum well structure are asymmetrical in refractive index distribution. It is known that when a window region in which the active layer is disordered is not provided, light intensity distribution at an outgoing end face of the semiconductor laser, i.e., a near-field image or pattern (NFP) is asymmetrical with respect to the active layer. The far-field pattern is expressed as a Fourier transform of the near-field pattern, so that the far-field pattern is symmetrical in both the direction normal to the active layer and the direction parallel to it. The center of the light intensity distribution is also placed on the normal extending from a light-emitting point of the laser outgoing end face.
However, in a semiconductor laser in which the cladding layers on the n and p sides are asymmetrical in refractive index distribution, is accompanied by a problem in that when a window region, in which the active layer is disordered, is provided, the center of the far-field pattern as viewed in the vertical direction is inclined about 0.5 to 3° toward the n-cladding layer side, and it is difficult to couple an external apparatus with the laser light.